The invention relates in general to parking brake systems for vehicles. More specifically, the invention relates to a parking control circuit that automatically applies a parking brake of a vehicle when the transmission of the vehicle is in the xe2x80x9cparkxe2x80x9d position and the engine of the vehicle is off.
Vehicle regulations in some jurisdictions require the parking brake of a bus be automatically applied whenever the door of the bus is open or the transmission of the bus is in the xe2x80x9cparkxe2x80x9d position and the engine of the bus is off. The application of the parking brake under these circunstances cannot be overridden except to allow for the servicing of the vehicle""s brake system. The regulations are intended to prevent injury in the event that a malfunction occurs in the transmission that might cause the bus to move when parked.
Conventional systems that provide such a feature utilize in the application of a parking brake either require the application of electrical power to a solenoid when the vehicle""s engine is running or the application of electrical power to a solenoid when the vehicle""s engine is off. The application of electrical power to an electrical solenoid when the engine is running can result in an inadvertent application of the vehicle""s parking brake should the application of electrical power be interrupted. The application of electrical power to an electrical solenoid when the engine is off results in a drain on the vehicle""s supply battery. The battery can become totally drained if the vehicle is parked for a long period of time, which can result in a release of the parking brake once the application of electrical power is interrupted to the solenoid.
In view of the above, it would be desirable to provide a parking brake control circuit that would engage the parking brake of a vehicle when the transmission of the vehicle is in the xe2x80x9cparkxe2x80x9d position and the engine of the vehicle is turned off, but would not require the constant application of electrical power to an electrical control solenoid as in the case of conventional systems.
A parking brake control circuit is provided that engages the parking brake of a vehicle when the transmission of the vehicle is in the xe2x80x9cparkxe2x80x9d position and the engine of the vehicle is turned off, without requiring the constant application of electrical power to an electrical control solenoid. The parking brake control circuit utilizes latching solenoid technology that eliminates the problems associated with conventional systems use of a constant voltage source. A time-delay relay is used to change the state of the latching solenoid. The time-delay relay is preferably activated through a circuit containing an engine oil pressure switch, a transmission park switch and a vehicle speed switch. Activation of the relay latches the latching solenoid in a position that allows the parking brake to be applied, without requiring the application of a constant voltage to the solenoid. Accordingly, the possibility of inadvertent brake application due to electrical failure is avoided.
More specifically, the parking brake control circuit incorporates an engine oil pressure switch including a first circuit input and a second circuit input, a transmission park switch including a first circuit input coupled to a first circuit output of the engine oil pressure switch and a second circuit input coupled to a second circuit output of the engine oil pressure switch, a first time delay relay having an input coupled to a second circuit output of the transmission park switch, a second time delay relay having an input coupled to a first circuit output of the transmission park switch, and a latching solenoid including a supply control input coupled to an output of the first time delay relay and an exhaust control input coupled to an output of the second time delay relay. The first circuit input of the engine oil pressure switch is coupled to an ignition circuit of the vehicle and the second circuit input of the engine oil pressure switch is coupled to a storage battery of the vehicle. A parking control valve is provided that includes a pilot control input, an air supply input and an air supply output. The air supply input of the parking control valve is coupled to an air supply of the vehicle and the air supply output of the parking control valve is coupled to a brake of the vehicle. The pilot control input of the parking control valve is coupled to a control port of the latching solenoid. In operation, the latching solenoid couples the pilot control port to the air supply of the vehicle when a signal is supplied to the supply control input, and couples the pilot control port to an exhaust port when a signal is supplied to the exhaust control input.
An override switch is also preferably provided to permit maintenance of the brakes. The override switch includes an input coupled to the ignition circuit of the vehicle and an output coupled to the input of the second time delay relay. Closing the override switch activates the second time delay relay to send a signal to the latching solenoid to couple the pilot control port to the exhaust port, thereby enabling the release of the parking brake.
Still further, in order to provide a greater measure of safety, a vehicle speed switch is provided that includes an input coupled to the second circuit output of the transmission park switch and an output coupled to the input of the first time delay relay. Accordingly, the first time delay relay is not activated unless the vehicle""s speed is below a predetermined speed. Thus, the parking brake will not be activated while the vehicle is still moving at a speed greater than the predetermined speed, regardless of whether the engine has been turned off and the transmission has been placed in the xe2x80x9cparkxe2x80x9d position.